Patients with kidney failure or partial kidney failure typically undergo hemodialysis treatment in order to remove toxins and excess fluids from their blood. To do this, blood is taken from a patient through an intake needle (or catheter) which draws blood from an artery located in a specifically accepted access location (for example, an arm, thigh, subclavian, etc.). The needle (or catheter) is connected to extracorporeal tubing that is fed to a peristaltic pump and then to a dialyzer which cleans the blood and removes excess water. The cleaned blood is then returned to the patient through additional extracorporeal tubing and another needle (or catheter). Sometimes, a heparin drip is located in the hemodialysis loop to prevent the blood from coagulating. By way of background, as the drawn blood passes through the dialyzer, it travels in straw-like tubes within the dialyzer which serve as semi-permeable passageways for the unclean blood. Fresh dialysate solution enters the dialyzer at its downstream end. The dialysate surrounds the straw-like tubes and flows through the dialyzer in the opposite direction of the blood flowing through the tubes. Fresh dialysate collects toxins passing through the straw-like tubes by diffusion and excess fluids in the blood by ultra filtration.
It is known in the art to use an optical blood monitoring system during hemodialysis, such as the CRIT-LINE® monitoring system which is sold by the assignee of this application. The CRIT-LINE® blood monitoring system uses optical techniques to non-invasively measure in real-time the hematocrit level of blood flowing through a hemodialysis system. In the CRIT-LINE® system, a sterile, single-use blood chamber is attached in-line to the extracorporeal tubing on the arterial side of the dialyzer. The blood chamber provides a viewing point for optical sensors during the hemodialysis procedure. Multiple wavelengths of light are directed through the blood chamber and the patient's blood flowing therethrough, and a photo detector (or array of photo detectors) detects the resulting intensity of each wavelength. The preferred wavelengths are about 810 nm, which is substantially isobestic for red blood cells, and about 1300 nm, which is substantially isobestic for water. A ratiometric technique implemented in the CRIT-LINE® controller, substantially as disclosed in U.S. Pat. No. 5,372,136 entitled “System And Method For Non-Invasive Hematocrit Monitoring”, which issued on Dec. 13, 1999 and is also assigned to the assignee of the present application, uses this information to calculate the patient's hematocrit value in real-time. The hematocrit value, as is widely used in the art, is the percentage determined by dividing the volume of the red blood cells in a given whole blood sample by the overall volume of the blood sample. The CRIT-LINE® system can also measure, optically, the oxygen saturation level in the blood flowing into the dialyzer.
In a clinical setting, the actual change in blood volume occurring during hemodialysis can be determined, in real-time, from the change in the measured hematocrit. Thus, a hemodialysis blood monitoring system which uses optical techniques, such as the CRIT-LINE® monitor, is able to monitor non-invasively, and in real-time, the patient's hematocrit level and in turn the accurate percentage change in blood volume during a hemodialysis treatment session. The ability to monitor change in blood volume facilitates safe, effective hemodialysis.
Hemoglobin is the iron containing oxygen transport contained in red blood cells, but constitutes less than 50% of the total content of the typical red blood cell. A decrease in hemoglobin, with or without an absolute decrease of red blood cells, can lead to anemia. The measured hematocrit (HCT), i.e. the portion of blood volume occupied by red blood cells, is typically about three times the hemoglobin (Hgb) level. It is widely assumed that HCT=2.941×Hgb at sea level. For example, when common blood tests are performed, a hemoglobin level measured at 17 grams per deciliter would normally correspond to a hematocrit (ratio of red cell volume to total blood volume) slightly less than 51 percent.
Hemoglobin levels can be measured in vitro directly from a patient's blood sample. When a blood sample is taken to a laboratory for lab work, direct hemoglobin measurements commonly require the lysing of red blood cells to free the hemoglobin into solution. The concentration of the hemoglobin in solution is then measured using assay techniques. Physicians typically monitor the Hgb level of anemic patients, and prescribe medication or other therapeutic care based at least in part on the patient's Hgb level.
Since optical hemodialysis blood monitoring is an in vivo process, it is important to maintain the integrity of the red blood cells flowing through the hemodialysis circuit during the monitoring process. Therefore, optical hemodialysis blood monitoring systems estimate hemoglobin levels based on the optically measured hematocrit (i.e., HCT=2.941*Hgb). The accuracy of the estimated hemoglobin levels is thus dependent upon the accuracy of the optically measured hematocrit.
Each optical blood monitoring system is calibrated for HCT by the manufacturer in-house before it is shipped. To calibrate, the manufacturer typical compares the system's output against in vitro lab work for the same blood sample. Typically, blood used for calibration is from a blood bank, and is preserved in a long-term preservative and anticoagulant, such as citrate phosphate dextrose (CPD) which includes nutrients to feed the blood. A cell counter, e.g. a Coulter counter, is normally used to determine the base line hematocrit values for calibrating purposes. The general formula for hematocrit is:
                    HCT        =                                            (              RBC              )                        ×                          (              MCV              )                                V                                    Eq        .                                  ⁢                  (          1          )                    where RBC is the red blood cell count in the blood sample, MCV is the mean cell volume of the measured red blood cells and V is the total volume of the sample. To measure hematocrit using a cell counter, a sample of blood is drawn from the patient into a test tube. The cell counter draws a metered volume (V) of the blood sample and red blood cells are literally counted as they drop through a small diameter pipette within the cell counter. This determines the red blood cell count (RBC) in the above Eq. (1). The average mean cell volume (MCV) is measured by running an electrical current through a designated area of the pipette. The size of the blood cell correlates to the amount of electrical current passed. Through mathematical means within the cell counter, the measured RBC and MCV values are used to determine the hematocrit per the Eq. (1).
An optical blood monitoring system, such as the CRIT-LINE® monitoring system, is calibrated against the results of the cell counter (or other in vitro method) by adjusting a constant in a mathematical ratiometric model that runs on its controller. The mathematical ratiometric model for determining the hematocrit value can be represented by the following equation:
                    HCT        =                  f          ⁡                      [                                          ln                ⁡                                  (                                                            i                      800                                                              I                      0800                                                        )                                                            ln                ⁡                                  (                                                            i                      1300                                                              I                      01300                                                        )                                                      ]                                              Eq        .                                  ⁢                  (          2          )                    where i800 is the light intensity of the photo receiver at 810 nm, i1300 is the light intensity of the photo detector at 1300 nm and I0800 and I01300 are constants representing the light intensity incident on the blood accounting for losses through the blood chamber. The function f is a mathematical function which has been determined based on experimental data to yield the hematocrit value. In the above Eq. (2), the constants I0800 and I01300 are unknown values which can be adjusted for calibration. In addition to being used to measure hematocrit (HCT), the measured light intensity at the 810 nm wavelength is used in the CRIT-LINE® monitor to determine the oxygen saturation level. Therefore, for purposes of calibration, it is desirable to keep the constant I0800 at a predetermined value. The constant I0800 can be estimated by measuring i800 through a blood chamber full of normal saline. Since no red cells are present, the measurement of i800 is approximately equal to I0800. The monitor is thus calibrated by adjusting the constant I01300 in order that the hematocrit value (HCT) determined by Eq. (2) matches the hematocrit value HCT determined by the cell counter in Eq. (1). This method is quite accurate and repeatable for measured hematocrit calibration.
However, actual blood draws in a clinic are preserved in potassium ethylene diamine tetra acetic (K3EDTA), which is a short-term preservative that does not change the morphology and optical characteristics of the red blood cells in the same manner as the long-term preservative CPD most often used to validate calibration by the manufacture. To account for this difference in preservatives, the prior art has often estimated the hemoglobin value from the measured hematocrit using a linear relationship (i.e. a slope of 3.000) with a positive offset correction of one hematocrit unit. For many applications to date, this estimation has been adequate.
A primary object of the invention is to better compensate, when measuring hematocrit values (HCT) and predicting hemoglobin values (Hgb) from the measured HCT, for differences between preservatives used during the calibration process and preservatives used in a clinical setting. Another object of the invention is to implement such improved compensation in an optical hemodialysis blood monitoring system, thereby providing more consistency between optically measured hematocrit and estimated hemoglobin values and those measured in vitro from a blood sample taken in a clinic.